By Maryn McKenna, Slate

The vicious virus has re-established itself in the South, and mosquitoes are carrying it north.

In the autumn of 1885, people in Austin, Texas, began to feel sick. One after another, they developed a chill and then a soaring fever. They vomited and broke out in rashes. Their most distinctive symptom was agonizing pain behind their eyes and in the bones of their arms and legs. And when the fever subsided, lack of appetite and deep exhaustion left them unable to work for weeks or months.

This week the global health community is mourning the violent loss of nine workers who were killed while administering vaccines in Pakistan as part of a national polio vaccination program.